This invention relates in general to magnetic recording and, more particularly, to apparatus for maximizing the ability of a recorder to overwrite, and thereby erase, pre-recorded signal information.
2. Description Relative to the Prior Art
The invention is cast in the environment of the digital computer art: Although terms such as "bit length" and "wavelength" are used widely in the recording arts, a substitute term "flux change length" (FCL) is adopted herein for purposes of the present description, since such term most clearly addresses the workings of the invention. It will be understood that "flux change length" (FCL) shall be interpreted to mean the distance that occurs between successive data-representative flux changes within a recording medium. The term "data", in turn, shall mean information relating to and forming part of a bit-stream, and does not correspond to such signals as pilots, tracking signals and the like.
Tremendous effort is now--and has for sometime been--underway to pack more and more digitized signal information onto the surface of a magnetic recording medium such as a floppy disk. To pack signal information onto a magnetic recording surface is synonymous with saying that the flux change length FCL of signals so recorded shall be extremely small. To prevent demagnetization fields from knocking down short FCL signals, and/or producing excessive pattern-dependant bit shift, the recording medium should be of high coercivity. In order to record signal information onto a medium of high coercivity, however, a fairly intense field within the gap of the record head so employed must be used, albeit that such field will oftentimes cause pole tip saturation of the record head. As is known, pole tip saturation works to enlarge the effective gap of the record head and, most significantly, causes the edges of the effective gap to become fuzzy. Attendantly, two phenomena, both related to media coercivity, work to prevent effective overwriting atop pre-recorded signal information: 1. the "fuzzy" gap prevents sharp erasure-conducive transitions of and within the field associated with the overwriting signal and 2. the high coercivity of the recording medium, by its very nature, precludes easy erasure of the pre-recorded signal information.
In the current state of the digital computer art, modified frequency modulation (MFM) recording is in vogue: depending on the order of the recorded bits, f, 1.5 f, and 2 f signal components . . . where f corresponds to signal frequency . . . are processed. (In a typical MFM recording, a ONE, for example, calls for a "transition" for each bit, and a ZERO does not . . . except in the event the ZERO is followed by another ZERO; this, therefore, results in the production of f, 1.5 f, and 2 f signal components.) Accordingly, it is not uncommon to record 1 f signals atop 2 f signals, or 2 f signals atop 1.5 f signals, or 2 f signals atop 1 f signals, etc. . . . all without benefit of a separate pre-erasure step. Of the various recording practices, however, it has been found that the recording of a 2 f signal atop a pre-recorded 1 f signal is least effective from the standpoint of erasing the 1 f signal. As has been determined, if the "erased" 1 f signal is not at least 26 db below what it was prior to the recording of the 2 f signal, excess errors will occur during playback of data. (It will be appreciated that reference to MFM coding is merely representative of state-of-the-art coding; and that the invention may be practiced with any form of coding.)